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<br />modeled. Additionally, the flood-hazard impacts associated with impaired channel <br />conveyance, which might be revealed by the consideration of bridge hydraulics <br />in the modeling, is simulated by the second flood scenario. <br /> <br />The second flood scenario analyzed assumes that flows from the Cornet Creek <br />canyon are unconstrained. Flood flows were f~ee to migrate under the influence <br />of topographic and roughness characteristics of the fan. <br /> <br />To assess the potential for flood flows on the eastern haH of the fan, a <br />deta il ed ana lys is of the hydraul i cs at th e apex of thE! fan was also conducted. <br />A detailed description of the fan apex hydraul ic analysis is presented in <br />Reference 9. <br /> <br />Flood events considered in the analysis 'Included both clear..water floods and <br />variable sediment concentration mud flows of 10-, 50-, 100-, and 500-year return <br />periods. <br /> <br />To simulate the topographic characteristics of the Cornet Creek alluvial fan two <br />gri del ement networks were deve loped for use in the 11UDFLOW anal ys is. One <br />network was created to analyze the hydraulics of flood flows on the alluvial fan. <br />It was developed based on mapping of 1:1,200 scale and 2-foot contour interval <br />(Reference 10). Each grid element represe1ts a square of the fan with sides of <br />300 feet. The second network was used to provide additional detail on the <br />hydraulic characteristics at the apex of the fan. It was developed based on a <br />topographic survey of the fan apex (Reference 11). Each el ement of the fan apex <br />grid system represents a square with sides of 25 feet. <br /> <br />Individual grid elements were assigned an average elevation and flow roughness <br />value. In grid elements containing the stream channel, the channel reaches were <br />defined by average values of width, depth and length. Physical constraints to <br />flow in each grid element were modeled by inclusion of appropriate flow width <br />or flow area reduction factors. Flow constraints included buildings, retaining <br />walls, or other physical features. <br /> <br />Average Manni ng' s n roughness co,effi c i ents were ass i gned to the channel reaches <br />and to the floodplain grid elements. Values for the channel ranged from 0.030 <br />to 0.088. Roughness coefficients for the floodplain grid varied from 0.035 to <br />0.085. A laminar flow roughness coefficient (K) for overland flow was assigned <br />an average value of 160. The coefficient K was adjusted as a function of the <br />Manning's n. A detailed discussion of the K parameter and its relationship <br />to Manning's n is presented in a separate report (RefE!renCe 12). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />4.1 Floodplain Boundaries <br /> <br />For Cornet Creek, flood boundaries were delineated using topographic maps at a <br />scale of 1:1,200 with a contour interval of 2 feet (Reference 10). Delineation <br />of areas of inundation, depth, and velocity was made based on output results of <br />the MUDFLOW model; some interpretation of output data was necessary. Where <br />hydraulic parameters of depth and velocity varied substantially between <br />contiguous grid elements, plotted contours were evenly centered on their <br />boundary. The 1 imits of i nundat"i on were corl'e 1 ated to phys i ca 1 fl ow barr i ers <br />